sometimes one of the pads will fall out of line with the rim. i.e., they don't run on the same plane exactly so you can just turn it slightly clockwise or counter clockwise to make the noise stop. I'm not talking about "towing in" since it just started happening all the sudden. I guess what I'm trying to convey is that one might have rotated CW or CCW if the attaching post is the pivot point.

They might even look correct to the eye, but even 1mm off can make a difference with vibrations sometimes. In any case, rotating the pads a little bit might make it stop, without affecting braking power; just be sure none of the pad is rubbing the tire, but you knew that part.

Steal a fingernail emery board from your mother, wife, girlfriend or sister. It's nice and flat, easy to use, and has just the right grit - also cheap. Use that to dress the surface of your brake pads. MOST of the time, that'll fix it.

I clean the rims ever so often with a rag dampened with water and Simple Green. I make sure that I apply pressure in the area where the brake pads hit the rims, as I rotate the wheel. Then I take the rag and clean the brake pads.

If it has suddenly started-then possible contamination. Clean the blocks with emery and scrub the rims with a fine grade of rubbing paper or rough cloth. Then check alignment. Then ride through a puddle to lubricate the blocks. All have worked for me with varying results.